The present invention relates to an organic electroluminescent device (organic EL device) wherein an organic layer having a luminescent region is provided between an anode and a cathode.
Lightweight, highly efficient flat panel displays, for example, for picture display of computers and television sets have been extensively studied and developed.
Since Braun tubes (CRT) are high in luminance and exhibit good color reproducibility, they are most widely employed for display at present. Nevertheless, the problems are involved in that the tubes are bulky, heavy and high in power consumption.
For lightweight flat panel displays of high efficiency, there have been available liquid crystal displays of the active matrix drive type. However, liquid crystal displays have the problems that their angle of visible field is narrow, they do not rely on spontaneous light and thus, need great power consumption for back light when being placed in a dark environment, and they do not have a satisfactory response to high-speed video signals with high fineness which have been expected to be in use in future. Especially, a difficulty is involved in making a liquid crystal display with a large picture size, with the attendant problem of its high fabrication costs.
As a substitute therefor, a display of the type using a light-emitting diode may be possible, but such a display is also high in fabrication costs, coupled with another problem that it is difficult to form a matrix structure of light-emitting diodes on one substrate. Thus, when being considered as an alternative for a low-cost display used in place of Braun tubes, this type of display still has a great problem to solve before putting to practical use.
As a flat panel display which has the possibility of solving these problems, attention has been recently paid to organic electroluminescent devices (organic EL devices) using organic luminescent materials. More particularly, by using organic compound as a luminescent material, it has been expected to realize a flat panel display, which makes use of spontaneous light, has a high response speed and visibility without depending on an angle of field.
The organic electroluminescent device is so constructed that an organic thin film, which contains a luminescent material capable of emitting light through charge of an electric current, is formed between an optically transparent anode and a metallic cathode. In the research report published in xe2x80x9cApplied Physics Lettersxe2x80x9d, Vol. 51, No. 12, pp. 913-915 (1987), by C. W. Tang and S. A. VanSlyke set forth a device structure (an organic EL device having a single hetero structure), which has a double-layered structure including, as organic thin films, a thin film composed of a hole transport material; and a thin film composed of an electron transport material and wherein luminescence occurs by re-combination of holes and electrons injected from the respective electrodes into the organic films.
In this device structure, either of the hole transport material or the electron transport material serves also as a luminescent material. Luminescence takes place in a wavelength band corresponding to the energy gap between the ground state and the energized state of the luminescent material. When using such a double-layered structure, a drive voltage can be remarkably reduced, along with an improved luminescent efficiency.
Thereafter, there has been developed a three-layered structure (organic EL device having a double hetero structure) of a hole transport material, a luminescent material and an electron transport material as set out in the research report of C. Adachi, S. Tokita, T. Tsutsui and S. Saito, published in xe2x80x9cJapanese Journal of Applied Physicsxe2x80x9d, Vol. 27, No. 2, pp. L269-271 (1988). Moreover, a device structure comprising a luminescent material contained in an electron transport material has been developed as set out in the research report of C. W. Tang, S. A. VanSlyke and C. H. Chen published in xe2x80x9cJournal of Applied Physicsxe2x80x9d, Vol. 65, No. 9, pp. 3610-3616 (1989). Through these researches, it has been proven the possibility of luminescence of high luminance at low voltage, thus leading to very extensive studies and developments in recent years.
Organic compound used as a luminescent material are considered to be advantageous in that because of their diversity in kind, a luminescent color can be arbitrarily changed by changing their molecular structure theoretically. Accordingly, it may be easier for it in comparison with thin film EL devices using inorganic materials to provide, via proper molecular design, three colors of R (red), G (green) and B (blue) having good color purities necessary for full color displays.
However, organic electroluminescent devices still have problems to solve. More particularly, a difficulty is involved in the development of a stable red luminescent device with high luminance. In an instance of such red luminescence attained by doping DCM[4-dicyanomethylene-6-(p-dimethylaminostyryl)-2-methyl-4H-pyran] in tris(8-quinolinol)aluminum (hereinafter abbreviated as Alq3) for use as a currently reported electron transport material, this material is not satisfactory as a display material with respect to both maximum luminance and reliability.
BSB-BCN, which was reported by T. Tsutsui and D. U. Kim in the Inorganic and Organic Electroluminescence Conference (at Berlin, 1996), is able to realize a luminance as high as 1000 cd/m2 or over, but is not always perfect with respect to the chromaticity for use as a red color for full color display.
Therefore, there is a need for a red luminescent device which is high in luminance, stable and high in color purity.
In Japanese Laid-open Patent Application No. Hei 7-188649 (Japanese Patent Application No. Hei 6-148798), it has been proposed to use a specific type of distyryl compound as an organic electroluminescent material. However, the intended luminescent color is blue, not red.
Therefore, there is a need for an organic electroluminescent device, which ensures high luminance and stable red luminescence.
Intensive studies have been made in order to solve the above problem, and as a result, it has been found that when using a specific type of distyryl compound as a luminescent material, there can be provided a highly reliable red luminescent device which is very useful for realizing a stable full color display of high luminance, thus arriving at completion of the invention.
More particularly, the invention relates to an organic electroluminescent device of the type which comprises an organic layer having a luminescent region provided between an anode and a cathode and which contains an organic material capable of generating luminescence by charging an electric current, characterized in that the organic layer contains, as an organic luminescent material, a distyryl compound represented by the following general formula (1), 
provided that, in the above general formula (1), R1, R2, R3 and R4 are, respectively, of groups which may be the same or different and represent an aryl group of the following general formula (2), 
provided that, in the above general formula (2), R5, R6, R7, R8 and R9 are, respectively, of groups which may be the same or different and represent a hydrogen atom, or at least one of them is a saturated or unsaturated alkoxyl group or an alkyl group).
The use, as a luminescent material, of a distyryl compound of the above general formula (1) enables to obtain stable red luminescence of high luminance, as well as to provide a device exhibiting electrically, thermally or chemically good stability.
The distyryl compound used in the organic electroluminescent device of the invention are described.
The distyryl compound represented by the general formula (1) and used as a luminescent material in the organic electroluminescent device of the invention may have at least one of molecular structures, for example, of the following structural formulas (3)-1, (3)-2, (3)-3, (3)-4, (3)-5, (3)-6 or (3)-7. These are all bis(aminostyryl) unsubstituted anthracene compounds having an alkoxy (or alkyl)phenyl group or an unsubstituted phenyl group. 
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.